Whittaker (WO 8706268, 1987) discovered that papain could efficiently catalyze the reaction of N-benzyloxycarbonyl-L-aspartic acid dibenzyl ester with L-phenylalanine methyl ester to form N-benzyloxycarbonyl-.alpha.-L-aspartyl (.beta.-benzyl ester )-L-phenylalanine methyl ester, an aspartame precursor. A disadvantage of this process is that the N-benzyloxycarbonyl-L-aspartic acid dibenzyl ester, which is used as the enzyme substrate, has to be prepared from N-benzyloxycarbonyl-L-aspartic acid and benzyl alcohol. Therefore, this process is complicated.
Condensation reaction of N-benzyloxycarbonyl-L-aspartic acid with L-phenylalanine methyl ester to form N-benzyloxycarbonyl-.alpha.-L-aspartyl-L-phenylalanine methyl ester, an aspartame precursor, has been described by Chen and Wang (J. Org. Chem. 52, 4589-4590, 1988). However, a lower yield (70 %) is obtained and more enzyme and longer reaction time (72 hours) are required in accordance with this process.
Furthermore, in the process of preparing N-benzyloxycarbonyl-.alpha.-L-aspartyl-L-phenylalanine methyl ester by papain, the product will be accumulated in the form of a cake and be easily hydrolyzed further by papain to give N-benzyloxycarbonyl-.alpha.-L-aspartyl-L-phenylalanine which is an undesirable by-product.